Biofuel demand is not the only market pressure being felt by US corn farmers. Much of the fast food that powers Americans – a $100 billion annual market – is indirectly made from corn as well, according to researchers in Hawaii.

Hope Jahren and Rebecca Kraft of the University of Hawaii purchased 486 servings of hamburgers, fries and chicken sandwiches from McDonald’s, Burger King and Wendy’s in Los Angeles, San Francisco, Denver, Detroit, Boston and Baltimore.

Back in the lab, they analysed the carbon isotope content of each serving. Previous research has shown that it is possible to determine whether an animal ate predominantly corn feed or grass from the ratio of C13 to C12 in its body tissue.

The pair found that 100% of the chicken in these three fast-food chains had been reared on corn alone. Some 93% of the beef came from cows that had been fed a corn-only diet. Just 12 burgers – all from west-coast Burger Kings – came from beef that had eaten something else…